This invention relates to a waveguide/microstrip mode transducer comprising a waveguide and a microstrip line which is operably coupled to the waveguide over a broad frequency range via a balanced transmission line. The transducer comprises an insulating substrate which extends along the waveguide in an E-plane thereof and further comprises two conductors which are respectively on opposite major surfaces of the substrate and which have three successive pairs of portions, the two portions of each pair being respectively on the opposite major surfaces, wherein the microstrip line comprises a first of the pairs of which the two portions are respectively a strip conductor portion and a ground plane conductor portion, wherein the balanced transmission line comprises a second of the pairs of which the two portions are each elongate and are each bounded by two transversely-spaced lateral edges both substantially spaced from the walls of the waveguide, and wherein the two portions of the third pair extend away from the second pair along the waveguide to opposite wall portions thereof.
Such a mode transducer is known from U.K. Patent Specification No. 1 494 024. In this mode transducer, a substrate supporting the microstrip line and the balanced line is arranged in a longitudinal plane of symmetry of a rectangular waveguide, parallel to the electric field lines of the fundamental TE.sub.10 mode in the waveguide. The balanced transmission line is connected at one end to the microstrip line by a balance-to-unbalance transformer (balun) comprising two slots extending into the ground plane of the microstrip line from an edge thereof that extends across the substrate perpendicular to the longitudinal axis of the waveguide. The slots are disposed one on each side of the strip conductor of the microstrip line, and the effective electrical length of each slot is approximately a quarter wavelength in the operating frequency range of the transducer. The conductors of the balanced line extend away from the microstrip line along the waveguide and in opposite directions away from the centre of the waveguide so that they are mirror images of one another, becoming progressively broader, and are coupled at R. F. to central portions of the broad walls of the waveguide.
The operation of the balun in this known mode transducer is related to the fact that the short-circuit at the closed end of each slot is transformed to an open-circuit at the mouth of the slot when the effective electrical length of the slot is exactly a quarter wavelength. R.F. current passing between the microstrip ground plane and the conductor of the balanced line connected thereto is thus constrained to flow through the ground plane longitudinally of the waveguide rather than towards the waveguide walls. However, when the operating frequency range is broad, for example a waveguide bandwidth (such as 26.5-40 GHz) or a major part thereof, the effective electrical length of each slot may differ substantially from a quarter wavelength over part of the frequency range. As a result, the impedance at the mouth of the slot will not then be very high, and the balun will not function in substantially the desired manner. In other words, the coupling between the microstrip line and the balanced line will be inherently frequency-dependent.
An improved waveguide/microstrip line mode transducer is proposed in U.K. Patent Specification No. 1 586 784. In this transducer, the microstrip line is coupled to the waveguide without an intermediate balanced line or the associated balun, and the conductor configuration is asymmetrical. The strip conductor of the microstrip line is connected by a further conductor extending therefrom to a first wall portion of the waveguide, providing an R. F.-connection therebetween. The ground plane of the microstrip line extends from a point opposite the connection of the strip conductor and the further conductor with a generally decreasing width, measured parallel to the electric field lines, to an opposite second wall portion of the waveguide and is R.F.-connected thereto, and also extends to the first wall portion with an edge of the ground plane so disposed as to form a transmission line with the trailing edge (as defined in the Specification) of the further conductor, this transmission line having a high impedance at said point in the operating frequency range. The invention is said to be based on the recognition that the conductor configuration of such a device need not be symmetrical and that the frequency-selective balance-to-unbalance transformer situated in the signal path and required as a result of the balanced line in the device known from U.K. Patent Specification No. 1 494 024 can also be avoided. However, difficulty has been experienced in reproducing the stated performance of a constructed embodiment of the later invention, and generally the performance of such an embodiment leaves something to be desired.
It may be noted that another kind of waveguide/microstrip mode transducer has been proposed by M. Arditi in Trans. IRE, Vol. MTT-3, March 1955, p 31. In this transducer, a single ridge extends along and across the waveguide from one broad wall thereof, the height of the ridge increasing progressively along the waveguide from zero to the height of the waveguide minus the thickness of a substrate carrying the microstrip line. The ground plane of the microstrip line is coplanar with and conductively connected to the broad wall of the waveguide opposite that from which the ridge extends, and the strip conductor of the microstrip line is conductively connected to the ridge. This can be both electrically and mechanically disadvantageous. The abrupt transition from the unbalanced microstrip line to the ridge waveguide and plain waveguide, in both of which propagation is normally in effectively a balanced mode, can cause some propagation along the waveguide on the outside as well as inside, which may result in loss or undesired coupling. The conductive connections between the ridge waveguide and the microstrip line, more especially the strip conductor thereof, tend to be fragile, and may easily be damaged by relative movement between the waveguide and microstrip line due, for example, to a change in temperature or to mechanical shock or vibration.